The basic performance features required in a silver halide emulsion for photographic use are high sensitivity, low fog, fine grain and high development activity. The silver halides are silver fluoride, silver chloride, silver bromide and silver iodide, but silver fluoride is usually not used in photographic emulsions because of its high water solubility. By combining the remaining three silver halides, endeavors have been made to improve the basic performance of the emulsion. Light absorption becomes stronger in the sequence silver chloride, silver bromide, silver iodide. On the other hand, development activity is reduced in this sequence, so that it is difficult to make light absorption and development activity compatible.
Klein and Moizaru disclosed mixed silver halide emulsions consisting of a silver halide core covered with a layer of different silver halides (concretely, a silver bromide nucleus, a primary layer of silver iodobromide containing 1% of silver bromide, and an outer layer of silver bromide), giving increased light sensitivity without impairing development activity (JP-B-43-13162). (The term "JP-B" as used herein means an examined Japanese patent publication.)
Koitabashi et al. disclosed that when a thin shell, having a thickness of 0.01 to 0.1 .mu.m, was applied to core grains of comparatively low silver iodide content, desirable photographic characteristics, such as an increase in covering power, were obtained (U.S. Pat. No. 4,444,877).
Such inventions, with the silver iodide content of the core part low, and accordingly the total silver iodide content low, are useful. However, when high sensitivity and high image quality are aimed at, a high iodination of the emulsion is indispensable.
Heightened sensitivity and heightened image quality when the iodine content of the core part is increased are disclosed in, for example, JP-A-60-138538, 61-88253 (EP-A-171238), 59-177535 (GB-A-2138963), 61-112142 and 60-143331 (the term "JP-A" as used herein means an unexamined published Japanese patent application).
The technical concept in common in this series of patents is that by having the iodine content in the core as high as possible, and the iodine content in the shell part low, the development activity and the light sensitivity are compatible.
However, the double structure grain based on this technical concept still has problems, i.e., due to sensitizing dyes, characteristic desensitization is large; when the sensitive material is maintained under high humidity conditions the sensitizing dyes are easily desorbed, etc.
Image formation by means of a silver halide color photographic material is particularly excellent, in comparison with other image formation methods, in the beauty of the image obtained. Furthermore, in order to extend this point of excellence and provide beautiful images, or in order to make possible more convenient operation of image recording, much work is being expended on improvement of silver halide color photographic materials.
The principal factor in raising image quality is improvement in graininess. With this object, by the use of so-called DIR compounds which release a development inhibiting material by reaction with the oxidized form of the color developer, improvement of the performance of the above-mentioned silver halide grains is achieved. However, DIR compounds are often accompanied by a decrease in sensitivity; they are of only limited use as a means for high sensitivity and also high image quality of photosensitive materials.
Another important factor to be mentioned in raising image quality is color reproduction. With this object, for example, methods giving color photographic materials possessing satisfactory color reproduction are disclosed in U.S. Pat. No. 4,686,175; the maximum sensitivity wavelength ranges of their blue-sensitive emulsion layer, green-sensitive emulsion layer and red-sensitive emulsion layer are prescribed, and further, they contain a diffusion development inhibitor or a precursor thereof which releases compounds by reaction with the oxidized form of the color developer. Thus, by changing the color temperature of the light source at the time of photographing there are few changes in color reproduction. This invention is excellent, but methods of obtaining excellent graininess are not mentioned.
Utilization of the interlayer inhibition effect is known for improving color reproduction. Taking the example of color negatives, by giving a development inhibition effect from the green-sensitive layer to the red-sensitive layer, the color development of the red-sensitive layer in white light exposure can be suppressed as compared with the case of a red exposure light. For a color negative paper system, in the case of exposure to white light, it reappears as gray on the color print, because graduation is balanced; the above-mentioned interlayer effect brings about a higher density of cyan color, on exposure to red, than in the case of gray exposure, and cyan color development on the print is suppressed to allow more highly saturated red reproduction. Similarly, a development inhibition effect on a green-sensitive layer from a red-sensitive layer gives highly saturated green reproduction.
As methods of boosting the interlayer effect, increase of the iodine content of the emulsion or use of a DIR compound are known. However, the DIR compounds known in the prior art are not entirely sufficient for the improvement of color reproduction. In cases in which the spectral sensitivity distribution overlap was increased, they had no effect in improving poor color reproduction.
A method of stipulating the width of the maximum sensitivity of spectral distribution of blue-, green- and red-sensitive silver halide emulsion layers, and of including a diffuse DIR compound, is disclosed in JP-A-59-131937. The object is to provide color photographic materials possessing small changes in color reproduction with changes in the color temperature of the light source when photographing, and moreover, high chroma color reproduction.
The present inventors tried combining various means as mentioned above, but were not able to obtain photosensitive materials which were satisfactory with regard to changes in the color reproduction owing to color temperature changes in the light source while photographing, and faithful half tone reproduction of high saturation and primary colors. This shows that when restricted to stipulation of maximum sensitivity breadth and utilization of diffusive DIR compounds alone, it is possible to obtain a reduction in the changes in color reproduction due to changes in the color temperature of the light source and an increased saturation for some colors, but it is not possible to faithfully reproduce the numerous colors, other than primary colors, which exist in the natural world, i.e., intermediate colored objects, skin colors, etc.